The present invention relates to electronically steerable antennas in general, and in particular to such antennas having a reflector assembly and phased array elements with alternate transmission/reflection operating modes.
Reflector type antennas are well known in the radar antenna art. Typically, such an antenna would have a dipole, slot, or a horn, called the primary feed aperture, radiating toward a large reflector called the secondary aperture. The large reflector is used to shape the radiated wave to produce the desired pattern. Reflector antennas generally provide a single beam and may be scanned only by mechanical means. One important advantage of the reflector type is that they are relatively inexpensive and can be utilized over wide mechanical scan angles. A variance of the reflector type of antenna is the lens antenna, which has a direct analog to an optical lens. Such lenses are used primarily for converting a spherical wave into a plane wave on the opposite side of the lens, the wave being refracted as it passes through the lens. These lenses may be designed using the principles of classical geometric optics.
Phased array antennas are also well known in the art. This type has an array of elements such as dipoles in which the signal feeding each dipole is varied in such a way that antenna beams can be formed in space and scanned very rapidly in azimuth and elevation. Phased array antennas are useful for tracking multiple targets or targets that possess great speed as the beam can be steered electronically rather than mechanically, as in the case of the reflector or lens antennas. In addition, phased array antennas can simultaneously track a plurality of targets by producing time-shared radar beams, such a feature is extremely difficult with mechanically scanned reflector or lens assemblies. Furthermore, the conventional reflector antenna has little or no side lobe or beam shape control while phased arrays may be designed with adaptive side lobe and beam shape control and hence can achieve a highly superior performance characteristic.
With respect to military aircraft, fire-control radars are used to aid the pilot with target detection, tracking, and aiming of rockets, missiles and other weapons. Such radars rely on their antennas to provide early detection and precision tracking of a plurality of threats and targets over an extremely wide angle of coverage. Additionally, given the high speed of today's modern aircraft, high gain of the antenna system is essential for early detection. The invention disclosed herein combines the advantages of both a phased array antenna and a reflector and lens antenna to satisfy the need of rapid beam scanning with wide angular coverage and high gain in a single antenna assembly.
Prior work in this area includes U.S. Pat. No. Re. 28,217 which discloses an electronically steerable antenna formed by an array of separate reflector units of controllable electrical path length. The units each receive energy from a source which is reflected at a phase corresponding to the electrical path of the corresponding unit. Also, U.S. Pat. No. 4,070,678 discloses a wide angle scanning antenna assembly including a switching matrix and a spherical electromagnetic lens. In addition, U.S. Pat. No. 3,755,815 teaches a scanning antenna employing a phased array antenna directing electromagnetic energy through a non-planar lens. While each of these patents is suitable for its intended purpose, neither patent combines the features of a reflector antenna with a phased array antenna to produce configuration suitable for use with a fire control radar.